Refers to baz which MUST be a sub-TLV (or in a group) of bar,
which is itself a sub-TLV (or in a group) of Foo.

This syntax needs to be compatible with the users file, sql, etc.

The proposal is to leverage the OID syntax again, this time by allowing partial OIDs:

Foo.bar.baz = 5
.stuff = 6

Here stuff refers to the previously used attributefoo.bar.baz.
The .stuff says it's in the same parentage as Foo.bar.baz,
i.e. foo.bar. If you want to go up a parentage, you could do
..stuff, which would say it's parented from foo.